how can I get pure white color in blender?
There are three main issues that prevent your stone from getting a brighter color:




Dim lighting: your light sources aren't strong enough to produce white colors, even from white materials. The best example here is the floor, which has a simple 0.8 gray material. If the floor is gray, there's no reason your stone will be white. Increase light intensity to fix this.
The wave texture produces some pretty dark bands, that will darken the entire material in any blending mode where dark colors darken the overall result (including the Overlay mode you're using). I put the wave texture through a color ramp to map the black into mid-light gray to fix this.
Even after moderating the wave texture, it helps to also reduce its influence in the following overlay node by toning down the Factor to 0.25.
And lastly, I added an RGB curves node to lighten the overall color as well.


On a general note, it's worth mentioning that you won't get a "pure white" color with so many bumps and overlayed textures that darken the overall result (Voronoi, Wave). But that's fine, because unless we're talking about a very smooth surface - like our floor here (instead of an ordinary natural stone), a perfectly white color will be unrealistic.